HCE Limited – Ioan Howells Consulting Civil & Structural Engineers SAB Seminar - Consultant’s Perspective 16 th May 2019
Introduction SUDS Requirements – The Four Pillars SUDS Change In Legislation – Beneficial? Impact Upon Stakeholders Homeowners/Self Builders Small Developers (2-10 unit Typical Developments) Larger Developers Agricultural Users (e.g. Farmers) Case Study 1 – Single Unit Development Case Study 2 – Mixed Use Development Case Study 3 – Agricultural Development Summary 1
SUDS Requirements – The Four Pillars 2
SUDS Change In Legislation – Beneficial? Since 07 th January 2019 developments with construction area of 100m 2 or more will require sustainable drainage to manage on-site surface water. There has been a growing concern by approving bodies in England and Wales that not enough is/has been done historically at the masterplanning stage of developments. Wales has taken the lead via the recent change in legislation thereby enforcing consideration of SUDS at the planning stage. Government/advisory bodies are concerned about increase in flooding events, pollution from uncontrolled run-off and the lack of groundwater recharge due to the increase in urbanisation with positive drainage systems not replicating natural greenfield drainage systems. Concern by some stakeholders that the change in regulation will significantly increase costs for development. At the moment we are in the early stages of implementation (first 5 months) so the true effect and impact is still being assessed. Need For Balance? 3
End User/Client Concerns CONCERNS RAISED – REAL OR PERCEIVED? Another Additional Cost To Be Borne Increase In Development Risk SUDS Drainage Solutions Cost More Than Traditional Solutions SUDS Require Large Areas Of Land Use Required Disproportionate Burden For Small Developments/Single Residential Units 4
Impact Upon Stakeholders Single Dwelling/Self Builders No Net additional cost to development but increase financial risk due to design costs being required upfront, prior to securing Planning. For small single unit developments not cost effective for drainage design to be separated. Therefore developer incurs full design costs early! Due to size of development and available land use – SUDS solutions can be limited but can also driven by client. Cost Savings from SUDS solution limited due to scale of development Minor Developments (<10 Units, Small Commercial) No Net additional cost to development but increase financial risk due to design costs being required upfront. Although proportionally this is much less compared to single unit developments. Full array of SUDS solutions are usually available. The challenge is changing perception/mindset of developers to consider wide array of SUDS solutions and potential benefits that good schemes bring. A SUDS solution is usually cheaper than a traditional drainage system (10% Considered Achievable). Large Areas Of land Use Required – No net increase in land use required. However there has been a general increase 5
Impact Upon Stakeholders Major Developments (>10 Units, Large Commercial) No Net Additional Cost For End User. Minor Increase In Development Risk – Yes but considered much less impact as a result of the scale of typical developments. Larger developers usually have available funds set aside for obtaining planning. SUDS solution can be cheaper than a traditional positive drainage system (10% Considered Achievable). Implementation of a good SUDS solution should not necessarily require additional areas of land use. A more balanced wider array of SUDS solutions are possible on larger developments and should be reviewed/implemented. Change in legislation is considered positive for such development ensuring early implementation during the masterplanning phase. This will benefit large developments by enhancing the communal areas and improve bio-diversity whilst improving water quality and groundwater re-charge. Agricultural Developments Additional Engineering Design Costs Incurred To Secure Planning & Increase In Risk Larger Impact Upon End Users - Agricultural Developments have historically had less planning restrictions compared to residential/commercial developments. Agricultural buildings are getting bigger! A typical 400 cattle shed can equate to similar roof drainage runoff from around 25-30 houses. Managing SW runoff off is considered key to reducing localised flooding and fluvial contamination. Additional Capital Costs For Constructing Suitable Drainage Solutions, however these can be offset by implementation of RWH and Reuse. On farms there is always a need for water! 6
Case Study 1 – Single Unit Development St. Anne’s Vicarage Design Challenges Impermeable Site with Heavy Clay No Nearby Watercourse Within Burry Inlet Area Discharge To Combined Public Sewer Accepted But with Low Discharge Rate SUDS Design Features WAVIN Aquacell Attenuation Storage Cells With Flow Control Meter Used To Control Peak Volume Permeable Paving To Help Attenuate and Improve water quality of SW run-off Rain Gardens for bio-remediation to improve quality of amenity space Lessons Learned Even without source control small scale SUDS solutions can be used to improve habitat/bio-diversity, water quality, slow run-off and reduce run-off volumes 7
Case Study 2 – Mixed Use Development Cwrt Canna, Llangan – 13 Residential, 3 Commercial Units Design Challenges 3-4m cross fall across site Impermeable Soils At Shallow Depth Small Permeable Band at Circa 3-4m depth Site Area Quite Tight SUDS Design Features Incorporated Permeable Paving Used Throughout Except For Main Access Into Site Pond Used To Enhance Bio-Diversity & Water Quality But Also Attenuate Run-Off From Commercial Development Areas Up To 24hrs for extreme events Swales Used To Convey Water Through Development Filter Strips Used To Convey Water Around Boundary Deep Borehole Soakaways For Infiltration Green Roofs For Residential Units Rain Gardens To Be Used To Enhance Bio-Diversity & Water Quality Tree Pit At Centre of Residential Square Lessons Learned Early Design Engagement Essential To Achieve Quality SUDS Solution Consideration of All Aspects Of SUDS Solutions Will Create Flexible Design Solutions To Manage SW Run-Off Encouragement of RWH and Grey Water Schemes – Still Not Much Take Up 8
Case Study 3 – Agricultural Development New 122x32 Cattle Shed Carmarthenshire Design Challenges 3-4m cross fall across site (maximising Soakaway and attenuation Efficiency) Permeable Soils At Shallow Depth Located in adjacent field Volume of Run-Off- Roof Area ~0.6Ha equivalent to ~30-35 domestic roofs. No Obvious Surface Attenuation Systems & Client Did Not Want to Consider RWH. SUDS Features Ground Permeable At Shallow Depth. Therefore reasonable For Infiltration Use of weirs within soakaway to ensure (maximising infiltration and attenuation Efficiency achieved). Discharge to watercourse limited to less than 2l/s/ha Roof Area ~0.6Ha equivalent to ~30-35 domestic roofs Lessons Learned Peak Volume Control For Extreme Events Will Require Management Of Large Volumes RWH and re-use for such developments should be considered as part of holistic approach to SW run-off management. RWH can be quite cost effective with a reasonable payback period achievable. An accurate cost/benefit analysis can be carried out. 9
Summary No Net Additional Cost To Stakeholders More Timing Issue. Some Increase Financial Risk Due To Design Costs Being Required Upfront Prior To Securing Planning. Single Dwelling/Self Builders Impacted By Change In Legislation. Design costs for development are proportionally higher than for larger schemes and costs are incurred upfront resulting in increased risk. Minor & Major Residential Developments No Cost Impact By Change In Legislation. It is Considered Change Will Likely Improve The Quality Of Schemes Considerably By Early Engagement! Agricultural Sector Considered Most Impacted By Legislation Due To Historical Issues Related With Planning Process/Obtaining Planning Approval. 10
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